The present invention relates to battery separators and particularly to battery separators exhibiting an extended high electrical resistance profile over temperatures to 180xc2x0 C. or more.
Batteries separators may be prepared by various techniques, for example, by way of extraction, or by way of a multi-step annealing/stretching process. This latter process was discovered by the Celanese Plastics Company of Summit, New Jersey in the early 1970""s. A crystalline polymer, such as polypropylene is first extruded into a film under conditions which enhance stress in the molten polymer. It is desirable to anneal the film in an untensioned or low tensioned state to perfect the necessary crystalline structure. The precursor thus prepared is elongated in the machine direction to introduce a network of slit-like voids. The deformation process may be used to control the pore size and pore size distribution as well as the overall porosity. See Bierenbaum, H. S., Isaacson, R. B., Druin, M. L., and Plovan, S. G., Microporous Polymeric Films, I and EC PRODUCT RESEARCH AND DEVELOPMENT, Vol. 13, pp. 2-9, March, 1974.
As battery design requirements became more sophisticated, the characteristics of battery separators over various temperature ranges has also been refined. Fusible separators with suitable xe2x80x9cshut downxe2x80x9d characteristics are particularly desirable as is apparent from the most recent literature.
U.S. Pat. No. 4,650,730 to Lundquist et al. discloses a multi-ply polymeric sheet useful as a battery separator. Typically, the sheet includes a first layer in the form of a microporous sheet (unfilled) and a second, filled microporous sheet. The microporous component sheets are produced by an extraction process, then laminated together to form the structure, which will become non-porous at elevated temperatures. Note column 11, example 1. The claims specify a thickness of less than 10 mils per layer, various pore sizes and filler loadings. See also U.S. Pat. No. 4,731,304 to Lundquist et al.
U.S. Pat. No. 5,281,491 to Rein et al. is directed to a multi-ply unfilled sheet product reported to be useful as a xe2x80x9cshut-downxe2x80x9d battery separator. The product is formed by co-extrusion (blown film) followed by extraction. See Columns 6-8; examples 1-6. It is noted in Column 10 that uniaxial stretching can be used to impart porosity.
U.S. Pat. No. 5,240,655 to Troffkin et al. describes yet another possible process for making a multi-ply battery separator. The process therein described includes a first co-extrusion step, followed by cold (liquid nitrogen) stretching, followed by warm stretching, followed by annealing.
Japanese Patent Application Nos. 98394 and 98395 of Kurauchi et al. teach a porous film. Both documents refer to co-extrusion as a fabrication possibility, however, note that lamination of films is the preferred option, followed by heat treatment and two-step stretching to impart porosity.
U.S. Pat. No. 5,667,911 to Yu et al. teaches a process for making seamless, cross-piled battery separators. The method described involves extruding a tubular film, collapsing the film, annealing, cold stretching, hot stretching and heat setting to produce microporous membranes. The membranes are then spirally slit and subsequently laminated.
U.S. Pat. No. 5,565,281 to Yu et al. teaches a process not unlike the ""911 patent as applied to making a thin, bi-layer shutdown battery separator of high puncture strength. Particular parameters appear in the specification and claims. See also, U.S. Pat. No. 5,691,077 directed to making a thin tri-layer membrane including two outer polypropylene membranes sandwiching a microporous polyethylene membrane. Note Table 8, column 9.
U.K. Publication No. 2, 298,817 discloses a porous film prepared by forming a non-porous laminate, stretching the laminate to impart porosity, followed by heat treatment. See p. 9 and following. The laminate may be prepared initially by co-extrusion as set forth in example 1, p. 13 and following. A similar process to prepare A/B/A tri-layer films is described in Kokai 8-250097. Note working examples. See also European Publication No. 0 794 583 at p. 5, lines 48 and following. Note FIG. 1(c) thereof.
Additional tri-layer membranes are disclosed in Japanese Patent Application Nos. 8-266398 and 8-293612 and Kokai 10-154499.
As can be seen from the foregoing, there has been continuous refinement of battery separator preparation, particularly in connection with multi-layer separators. Early developments involve making a plurality of porous structures followed by lamination to complete fabrication. Thereafter non-porous films, separately prepared, were laminated together and the multi-layer non-porous structures were further processed to impart porosity to the product. Most recently multiple layers are co-extruded into a single non-porous structure which is processed into a porous structure thereby minimizing the number of processing steps needed to make the product.
So also, it is desirable to improve the thermal characteristics of battery separators, particularly with respect to xe2x80x9cshut-downxe2x80x9d properties desirable in the high performance market. Ideally, a membrane designed for service where a thermal shut-down is desired should radically increase its impedance at a first temperature threshold of 120-130xc2x0 C. or so and continue exhibiting increased impedance as long as it is possible, up to the crystalline melting point of the polymer or beyond at high rates of temperature increase. Some of the more recent literature in the field recognize such desirable characteristics.
Asahi Chemical Industry Co., Ltd. Discloses in Laid-Open Application No. 3-203160 a temperature resistant battery separator with a maximum impedance at least about 10xc2x0 C. higher than the temperature at which the impedance of the separator initially rises to 10 times its value at room temperature (R25). The porous membranes are prepared by way of extraction from high molecular weight polyethylene and maximum impedance temperatures at scan rates of 2xc2x0 C. per minute are reported to be up to about 25xc2x0 C. higher than the temperature at which impedance initially begins to rise.
U.S. Pat. No. 5,480,745 to Nishiyama et al., discloses co-extruded porous bi-layer films, where one layer is polypropylene and one layer is a mixture of polyethylene and polypropylene. The membranes are reported to exhibit a rise in impedance at about 130xc2x0 C. and a decay in impedance at about 170xc2x0 C.
It has been found in accordance with the present invention that membranes with enhanced resistance performance against temperature are prepared by rapidly quenching a molten film prior to imparting porosity to the separator.
There is provided in accordance with the present invention a battery separator formed of a microporous polyolefinic membrane generally capable of maintaining an electrical resistance greater than about 10,000 ohms-square centimeter at a temperature of at least about 185xc2x0 C. as measured at a scan rate of 60xc2x0 C. per minute. Typically the membrane is capable of maintaining an electrical resistance of greater than about 10,000 ohms-square centimeter at a temperature of at least about 185xc2x0 C. at a scan rate of 2xc2x0 C. per minute; while, preferably, membranes in accordance with the invention are capable of maintaining an electrical resistance greater than about 10,000 ohms-square centimeter at temperatures from about 130xc2x0 C. to at least 185xc2x0 C. as measured at a scan rate of either 60xc2x0 C. per minute or 2xc2x0 C. per minute. Most preferably the foregoing high resistance is maintained to 195xc2x0 C. or more, such as 200xc2x0 C. or more at scan rates of 2xc2x0 C. per minute or 60xc2x0 C. per minute.
The separator in accordance with the invention may be made from a variety of polymers including high density polyethylene, isotactic polypropylene or combinations thereof. Other polypropylenes and polyethylenes such as ultra high molecular weight polyethylenes may be employed. In the most preferred embodiments, multi-layer membranes are employed having at least one polypropylene layer and at least one polyethylene layer. Tri-layer membranes are particularly preferred.
In another aspect of the invention, there is provided a method of making a battery separator including the steps of: extruding a polymer film in a molten state in the form of a cylindrical parison having first and second surfaces; applying a low temperature fluid stream to both the first and second surfaces of the cylindrical parison, the low temperature fluid stream being operative to quench the molten polymer film such that it is in a substantially solidified state; followed by imparting microporosity to said polymer film to make the battery separator. Most preferably the process includes co-extruding a polymer film with at least one polyethylene layer and at least one polypropylene layer.
Following quenching of the polymer film in accordance with the inventive process, microporosity is most preferably imparted to the film by annealing the film to enhance its crystalline structure, elongating the film and heat setting it to provide dimensional stability. The step of elongating the film preferably includes elongating the film by at least about 20% at a low temperature (15-35xc2x0 C.) followed by further elongating the film at an elevated temperature (110xc2x0 C.-130xc2x0 C.) by at least about 100%. Relaxation upon heat setting is typically about 8% to about 15%.